Polymer foams are materials made by generating void spaces inside a bulk polymer, resulting in substantially reduced density. When these void spaces are interconnected, the material is characterized as open-celled. If these voids are discrete and not connected, the material is characterized as closed-celled. The nature of these cells and the cell size determine many properties of the polymer. For example, light weight and low thermal conductivity are the typical properties of a conventional foam. The density of conventional styrofoam is in the range of 0.02-0.2 g/cm.sup.3 with closed cells between 50-100 .mu.m in diameter.
When the cell size is less than 10 .mu.m, the foams are referred to as microcellular foams. Several remarkable properties have been noted for microcellular foams. First the strength/weight ratio for a closed-cell microcellular foam has been shown to be 5-6 times higher than for a macrocellular foam. Second, the high pore volume and high surface area of open-celled aerogels suggest applications as catalyst supports. The variability of the cell size and surface chemistry suggest novel applications as membranes or controlled release supports. Finally, when cell sizes are smaller than 0.040 .mu.m, the materials become transparent while retaining their low densities (0.05-0.10 g/cm.sup.3 ) and relatively low thermal conductivities.
The key to making a microcellular foam is to focus on the mode of phase separation. Phase separation in conventional foaming occurs when the bubble forms and inflates in a manner that is difficult to control resulting in non-uniform foams. Synthesizing a microcellular foam requires gaining significantly greater control over the phase separation process. The method by which the lowest densities and smallest pore sizes have been obtained is reaction induced phase separation with critical point drying. Critical point drying was first applied to foam drying of aerogels, and subsequently applied to an organic resorcinol-formaldehyde foam.
Probably the most versatile preparation technique involves thermally induced phase separation (TIPS) of polymer solutions. In this technique, a polymer solution is quenched in order to induce phase separation, either through liquid-liquid phase separation or polymer crystallization.
When the TIPS process results in the formation of a continuous polymer-rich phase, two additional processing steps can lead to a microcellular foam. First, the morphology of the phase-separated solution is preserved either through vitrification or crystallization of the polymer. This step preserves the small-scale morphology of the demixed solution. Next, the solvent is removed through freeze-drying or supercritical extraction.
The TIPS process is a general method whose primary requirement is polymer solubility. Low-density microcellular foams have been prepared with TIPS using many different polymers, including atactic polystyrene, isotactic polystyrene, poly(4-methyl-1-pentene), polyacrylonitrile, and water-soluble polymers such as (carboxymethyl)cellulose, poly(acrylic acid) and dextran.
One significant limitation of the commercially viable processes is that pores are produced that are generally closed-celled and poorly controlled in pore size and morphology.